A. Technical Field
The present invention relates to a water-absorbing composition and its use.
B. Background Art
In recent years, water-absorbing agents such as water-absorbent resins are widely used as components of sanitary materials, such as disposable diapers, sanitary napkins and incontinent pads, for the purpose of causing the water-absorbing agents to absorb much water. In addition, the water-absorbing agents such as water-absorbent resins are widely used not only for the sanitary materials, but also for the purpose of absorbing or retaining water, like water-retaining agents for soil and drip sheets for foods.
Known examples of the above water-absorbing agents are as follows: partially-neutralized and crosslinked poly(acrylic acids); neutralized graft polymers of starch-acrylic acid; saponified copolymers of vinyl acetate-acrylic acid ester; hydrolyzed copolymers of acrylonitrile or acrylamide, or crosslinked polymers of these hydrolyzed copolymers; and crosslinked polymers of cationic monomers.
And, in order to use the above water-absorbing agents for the above sanitary materials and so on, the water-absorbing agents are desired to have properties (absorption properties) such as high absorption capacity or rate upon contact with aqueous liquids (e.g. body fluids) and high absorption capacity or liquid permeability under load. And various water-absorbing agents have been proposed so far, which jointly have various properties among the above absorption properties and exhibit excellent performances when used for the sanitary materials.
Furthermore, similarly to the water-absorbing agents, absorbent articles comprising the water-absorbing agents and hydrophilic fibers (e.g. pulp) are also desired to exhibit the following performances: a large absorption capacity when aqueous liquids (e.g. body fluids) are injected into absorbent structures; a large absorption capacity under load; and excellent diffusing ability for the aqueous liquids to spread over the whole absorbent structure.
The diffusing ability for the aqueous liquids (e.g. body fluids) to spread over the whole absorbent structure is related greatly to capillary phenomena of the hydrophilic fibers. That is to say, the concentration of the water-absorbing agent (hereinafter referred to as xe2x80x9cresin concentrationxe2x80x9d) in the absorbent article mainly comprising the water-absorbing agent and the hydrophilic fibers is conventionally in the range of about 20 to about 50 weight %, and the aqueous-liquid diffusing ability of the absorbent article comprising the water-absorbing agent in this range of weight ratio is dominated by diffusion due to capillary phenomena of the hydrophilic fibers.
Therefore, the absorbent structure in the above sanitary material has problems in that in the case where an aqueous liquid is injected in a large quantity at once into the absorbent structure, the aqueous liquid leaks out from edge portions of the absorbent article, because the water absorption rate of the water-absorbing agent is slower than the diffusion rate of the aqueous liquid in the absorbent structure.
An object of the present invention is to provide: a water-absorbing composition such that portions containing the water-absorbing composition in a high concentration exhibit excessive-liquid permeation buffering effects in absorbent articles comprising the water-absorbing composition and hydrophilic fibers; and a use of this water-absorbing composition.
The present inventors made various studies to solve the above problems. As a result, they found that it is important to control the diffusion (to buffer the excessive permeation) of aqueous liquids when the water-absorbing composition comes into contact with the aqueous liquids in a state where the basis weight (resin concentration) of the water-absorbing composition is high and where particles of the water-absorbing composition closely cohere by external force. The above diffusion control (buffering of excessive permeation) of the aqueous liquids can be evaluated by introducing a new physical property value referred to as a liquid permeation buffering index. Then, the inventors completed the present invention by finding that the above problems can be solved by a water-absorbing composition which exhibits the following properties:
(1) The water-absorbing composition exhibits excellent water absorption properties in absorbent articles comprising the water-absorbing composition and hydrophilic fibers.
(2) The water-absorbing composition has a high liquid permeation buffering index, and can prevent the diffusion (buffer the excessive permeation) of aqueous liquids in the case where the resin concentration is high.
(3) If high resin concentration portions are placed on the periphery of the absorbent article as recited in (1) above, then the aqueous liquid can be prevented from leaking out from edge portions of the absorbent article when an aqueous liquid is injected in a large quantity at once into the absorbent article.
Accordingly, the present invention relates to a water-absorbing composition, which comprises a water-absorbent resin and an excessive-liquid permeation buffer, wherein the water-absorbent resin is a product by a process including the step of further crosslinking the surface vicinity of a crosslinked polymer, with the water-absorbing composition being characterized by having a liquid permeation buffering index of not less than 0.4 and exhibiting an absorption capacity of not less than 25 g/g in absorbent structures under load, and further having an average particle diameter in the range of 200xcx9c600 xcexcm, wherein the ratio of particles having particle diameters smaller than 106 xcexcm in the water-absorbing composition is not more than 5 weight %. The present invention further relates to an absorbent article, which comprises the above water-absorbing composition according to the present invention.
Incidentally, the above water-absorbing composition according to the present invention has an average particle diameter in the range of 200xcx9c600 xcexcm, and the ratio of particles having particle diameters smaller than 106 xcexcm in the water-absorbing composition is not more than 5 weight %.
These and other objects and the advantages of the present invention will be more fully apparent from the following detailed disclosure.